A karyosome or karyosphere is a dense bundle of chromatin inside the nucleus of a cell within an organism. These bundles are joined together in a limited nuclear volume, but this only happens when the cell is not undergoing meiotic division. Research suggests that within its bundles there is an absence of RNA synthesis occurring in the karyosphere. [1] This suggests that RNA is actively being carried out within meiosis, especially throughout the first and second prophase of meiotic division.
Karyosomes are formed in several animal classes and have a role in oogenesis, a vital process in female gamete development. The gametes can only develop if the mother has oocytes, [1] that are rich in cytoplasm, maternal proteins and contain karyosomes.
Karyosomes are known to be very organized. Any mutation that alters the formation and alignment of karyosomes can lead to defects of an oocyte. Defects can also occur with the absence of nucleosomal histone kinase-1. NHK-1 is a kinase that is essential for all karyosomes, for their maintenance and overall formation. NHK-1 is conserved from nematodes to humans. In the absence of NHK-1, karyosome patterns will fall apart within the female's oocyte, and this disruption will lead to problems previously mentioned. [2] There are other causes of mutations, however they are not fully understood.
The formation of karyosomes is still unclear, but NHK-1 substrates may help understand how the karyosome forms during female meiosis. [2]
Even though the molecular pathway for this process is to be confirmed, it is known that karyosomes tend to formulate at different stages for different organisms. In insects this occurs in the diplotene stage, a prolonged segment that crosses over genes from two different cells, and aids in the creation of gametes. For example, this occurs within drosophila oocytes . The formation of karyosomes during this period are based on a ratio. This ratio is dependent on the potential size of an organism, meaning that typically larger organisms will have bigger karyosomes. In other words, a typical drosophila oocyte is 20 μm in diameter with karyosomes each expected to be about 1 μm in diameter.
As oogenesis continues, the karyosome is typically covered by a sheath, known as a capsule. These capsules are created from the interaction between nuclear structures, the nuclear membrane and chromosomes. Karyosomes can also form without a capsule, such as in drosophila. The function and molecular composition of karyosome capsules are quite unknown. It has been suggested that these [3] capsules may be a storage site for nuclear ribonucleoprotein particles. Therefore, the absence of this capsule within certain organisms may demonstrate a reduction in small nuclear proteins and RNA.
Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote.
The egg cell, or ovum, is the female reproductive cell, or gamete, in most anisogamous organisms. The term is used when the female gamete is not capable of movement (non-motile). If the male gamete (sperm) is capable of movement, the type of sexual reproduction is also classified as oogamous. A nonmotile female gamete formed in the oogonium of some algae, fungi, oomycetes, or bryophytes is an oosphere. When fertilized the oosphere becomes the oospore.
A germ cell is any cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo meiosis, followed by cellular differentiation into mature gametes, either eggs or sperm. Unlike animals, plants do not have germ cells designated in early development. Instead, germ cells can arise from somatic cells in the adult, such as the floral meristem of flowering plants.
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division (mitosis/meiosis). There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).
An oocyte, oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell (PGC), which then undergoes mitosis, forming oogonia. During oogenesis, the oogonia become primary oocytes. An oocyte is a form of genetic material that can be collected for cryoconservation.
Cell growth refers to an increase in the total mass of a cell, including both cytoplasmic, nuclear and organelle volume. Cell growth occurs when the overall rate of cellular biosynthesis is greater than the overall rate of cellular degradation.
Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage.
Drosophila embryogenesis, the process by which Drosophila embryos form, is a favorite model system for genetics and developmental biology. The study of its embryogenesis unlocked the century-long puzzle of how development was controlled, creating the field of evolutionary developmental biology. The small size, short generation time, and large brood size make it ideal for genetic studies. Transparent embryos facilitate developmental studies. Drosophila melanogaster was introduced into the field of genetic experiments by Thomas Hunt Morgan in 1909.
Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two nuclei. Before karyogamy, each haploid cell has one complete copy of the organism's genome. In order for karyogamy to occur, the cell membrane and cytoplasm of each cell must fuse with the other in a process known as plasmogamy. Once within the joined cell membrane, the nuclei are referred to as pronuclei. Once the cell membranes, cytoplasm, and pronuclei fuse, the resulting single cell is diploid, containing two copies of the genome. This diploid cell, called a zygote or zygospore can then enter meiosis, or continue to divide by mitosis. Mammalian fertilization uses a comparable process to combine haploid sperm and egg cells (gametes) to create a diploid fertilized egg.
Sexual reproduction is an adaptive feature which is common to almost all multicellular organisms and various unicellular organisms, with some organisms being incapable of asexual reproduction. Currently the adaptive advantage of sexual reproduction is widely regarded as a major unsolved problem in biology. As discussed below, one prominent theory is that sex evolved as an efficient mechanism for producing variation, and this had the advantage of enabling organisms to adapt to changing environments. Another prominent theory, also discussed below, is that a primary advantage of outcrossing sex is the masking of the expression of deleterious mutations. Additional theories concerning the adaptive advantage of sex are also discussed below. Sex does, however, come with a cost. In reproducing asexually, no time nor energy needs to be expended in choosing a mate. And if the environment has not changed, then there may be little reason for variation, as the organism may already be well adapted. Sex also halves the amount of offspring a given population is able to produce. Sex, however, has evolved as the most prolific means of species branching into the tree of life. Diversification into the phylogenetic tree happens much more rapidly via sexual reproduction than it does by way of asexual reproduction.
Spermatocytes are a type of male gametocyte in animals. They derive from immature germ cells called spermatogonia. They are found in the testis, in a structure known as the seminiferous tubules. There are two types of spermatocytes, primary and secondary spermatocytes. Primary and secondary spermatocytes are formed through the process of spermatocytogenesis.
Nuclear DNA (nDNA), or nuclear deoxyribonucleic acid, is the DNA contained within each cell nucleus of a eukaryotic organism. It encodes for the majority of the genome in eukaryotes, with mitochondrial DNA and plastid DNA coding for the rest. It adheres to Mendelian inheritance, with information coming from two parents, one male and one female—rather than matrilineally as in mitochondrial DNA.
An oogonium is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female gametangium of certain thallophytes.
Endoreduplication is replication of the nuclear genome in the absence of mitosis, which leads to elevated nuclear gene content and polyploidy. Endoreplication can be understood simply as a variant form of the mitotic cell cycle (G1-S-G2-M) in which mitosis is circumvented entirely, due to modulation of cyclin-dependent kinase (CDK) activity. Examples of endoreplication characterized in arthropod, mammalian, and plant species suggest that it is a universal developmental mechanism responsible for the differentiation and morphogenesis of cell types that fulfill an array of biological functions. While endoreplication is often limited to specific cell types in animals, it is considerably more widespread in plants, such that polyploidy can be detected in the majority of plant tissues.
In developmental biology, the cells that give rise to the gametes are often set aside during embryonic cleavage. During development, these cells will differentiate into primordial germ cells, migrate to the location of the gonad, and form the germline of the animal.
Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. Chromosome segregation also occurs in prokaryotes. However, in contrast to eukaryotic chromosome segregation, replication and segregation are not temporally separated. Instead segregation occurs progressively following replication.
The meiotic recombination checkpoint monitors meiotic recombination during meiosis, and blocks the entry into metaphase I if recombination is not efficiently processed.
The fusome is a membranous structure found in the developing germ cell cysts of many insect orders. Initial description of the fusome occurred in the 19th century and since then the fusome has been extensively studied in Drosophila melanogaster male and female germline development. This structure has roles in maintaining germline cysts, coordinating the number of mitotic divisions prior to meiosis, and oocyte determination by serving as a structure for intercellular communication.
Resumption of meiosis occurs as a part of oocyte meiosis after meiotic arrest has occurred. In females, meiosis of an oocyte begins during embryogenesis and will be completed after puberty. A primordial follicle will arrest, allowing the follicle to grow in size and mature. Resumption of meiosis will resume following an ovulatory surge (ovulation) of luteinising hormone (LH).
The germ cell nest forms in the ovaries during their development. The nest consists of multiple interconnected oogonia formed by incomplete cell division. The interconnected oogonia are surrounded by somatic cells called granulosa cells. Later on in development, the germ cell nests break down through invasion of granulosa cells. The result is individual oogonia surrounded by a single layer of granulosa cells. There is also a comparative germ cell nest structure in the developing spermatogonia, with interconnected intracellular cytoplasmic bridges.